The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for current control. Merely by way of example, the invention has been applied to constant current control of power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 110, a secondary winding 112, an auxiliary winding 114, a power switch 120, a current sensing resistor 130, two rectifying diodes 160 and 168, two capacitors 162 and 170, and two resistors 164 and 166. For example, the power switch 120 is a bipolar transistor. In another example, the power switch 120 is a metal-oxide-semiconductor (MOS) transistor. In yet another example, the power conversion system 100 provides power to one or more light-emitting-diodes (LED) 199.
The auxiliary winding 114 can be used to extract information associated with an output voltage 150 on the secondary side so that the output voltage 150 can be regulated. When the power switch 120 is closed (e.g., on), the energy is stored in the transformer including the primary winding 110 and the secondary winding 112. When the power switch 120 is open (e.g., off), the stored energy is released to the output terminal. The voltage of the auxiliary winding 114 maps the output voltage 150. The auxiliary winding 114 and associated components (e.g., the resistors 164 and 166) generates a feedback signal 174 which can be determined based on the following equation:
                              V          FB                =                                            R              2                                                      R                1                            +                              R                2                                              ×                      V            aux                                              (                  Equation          ⁢                                          ⁢          1                )            where VFB represents the feedback signal 174, and Vaux represents a voltage 154 of the auxiliary winding 114. R1 and R2 represent the resistance values of the resistors 164 and 166 respectively.
For example, the switch 120 is associated with a switching period including an on-time period during which the switch 120 is closed (e.g., on) and an off-time period during which the switch 120 is open (e.g., off). As an example, in a continuous conduction mode (CCM), a next switching cycle starts prior to the completion of a demagnetization process associated with the transformer including the primary winding 110 and the secondary winding 112. Therefore, the duration of the demagnetization process (e.g., a demagnetization period) before the next switching cycle starts is approximately equal to the off-time period of the switch. For example, in a discontinuous conduction mode (DCM), a next switching cycle does not start until a time period after the demagnetization process has completed. In another example, in a critical conduction mode (CRM) (e.g., a quasi-resonant (QR) mode), a next switching cycle starts shortly after the completion of the demagnetization process.
FIG. 2 is a simplified conventional timing diagram for the flyback power conversion system 100 that operates in the continuous conduction mode (CCM). The waveform 202 represents the feedback signal 174 of the auxiliary winding 114 as a function of time, the waveform 204 represents a primary current 176 that flows through the primary winding 110 as a function of time, and the waveform 206 represents a secondary current 178 that flows through the secondary winding 112 as a function of time.
For example, a switching period, Ts, starts at time t0 and ends at time t2, an on-time period, Ton, starts at the time t0 and ends at time t1, and a demagnetization period, Tdem, starts at the time t1 and ends at the time t2. In another example, an off-time period is approximately equal in duration to the demagnetization period. In yet another example, t0≦t1≦t2.
During the on-time period Ton, the power switch 120 is closed (e.g., on), and the primary current 176 flows through the primary winding 110 and increases from a magnitude 208 (e.g., at t0) to a magnitude 210 (e.g., at t1) as shown by the waveform 204. The secondary current 178 is at a low magnitude 212 (e.g., approximately zero) as shown by the waveform 206. The feedback signal 174 keeps at a magnitude 214 (e.g., as shown by the waveform 202).
At the beginning of the demagnetization period Toff (e.g., at t1), the switch 120 is open (e.g., off), the primary current 176 is reduced from the magnitude 210 to a magnitude 216 (e.g., approximately zero) as shown by the waveform 204. The secondary current 178 increases from the magnitude 212 (e.g., approximately zero) to a magnitude 218 as shown by the waveform 206. The feedback signal 174 increases from the magnitude 214 to a magnitude 220 (e.g., as shown by the waveform 202).
During the demagnetization period Tdem, the switch 120 remains open, the primary current 176 keeps at the magnitude 216 (e.g., approximately zero) as shown by the waveform 204. The secondary current 178 decreases from the magnitude 218 (e.g., at t1) to a magnitude 222 (e.g., at t2) as shown by the waveform 206. The feedback signal 174 decreases from the magnitude 220 to a magnitude 222 (e.g., as shown by the waveform 202).
At the end of the demagnetization period Tdem (e.g., at t2), a next switching cycle starts before the demagnetization process is completed. The residual energy reflects back to the primary winding 110 and appears as an initial primary current 224 at the beginning of the next switching cycle.
As an example, the primary current 176 and the secondary current 178 satisfy the following equations:Isec_1=N×Ipri_1   (Equation 2)Isec_0=N×Ipri_0   (Equation 3)where Isec1 represents the secondary current 178 when the demagnetization period Tdem starts, and Isec_0 represents the secondary current 178 when the demagnetization period Tdem ends. Additionally, Ipri_1 represents the primary current 176 when the on-time period Ton ends, Ipri_0 represents the primary current 176 when the on-time period Ton starts, and N represents a turns ratio between the primary winding 110 and the secondary winding 112.
For example, the output current 152 is equal to an average of the secondary current 178 as shown by the following equation.
                              I          out                =                              1            2                    ×                      1            T                    ×                                    ∫              0              T                        ⁢                                          (                                                      I                                          sec_                      ⁢                      1                                                        +                                      I                                          sec_                      ⁢                      0                                                                      )                            ×                                                T                  dem                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          4                )            where Iout represents an output current 152 on the secondary side, T represents an integration period, Ts represents a switching period, and Tdem represents the duration of the demagnetization process within the switching period.
Therefore, the output current 152 satisfies the following equation:
                              I          out                =                              N            2                    ×                      1            T                    ×                                    ∫              0              T                        ⁢                                          (                                                      I                                          pri_                      ⁢                      1                                                        +                                      I                                          pri_                      ⁢                      0                                                                      )                            ×                                                T                  dem                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          5                )            
As shown in FIG. 1, the resistor 130, in combination with other components, generates a current-sensing voltage signal 172 which is related to the primary current 176. For example, the output current 152 can be determined according to the following equations:
                                          I            out                    =                                    N              2                        ×                          1                              R                s                                      ×                          1              T                        ×                                          ∫                0                T                            ⁢                                                (                                                            V                                              cs                        ⁢                                                                                                  ⁢                        1                                                              +                                          V                                              cs                        ⁢                                                                                                  ⁢                        0                                                                              )                                ×                                                      T                    dem                                                        T                    s                                                  ⁢                                                                  ⁢                                  ⅆ                  t                                                                    ⁢                                  ⁢                                  ⁢        or                            (                  Equation          ⁢                                          ⁢          6                )                                          I          out                =                              N            2                    ×                      1                          R              s                                ×                      1            K                    ×                                    ∑              1              K                        ⁢                                          (                                                                            V                                              cs                        ⁢                                                                                                  ⁢                        1                                                              ⁡                                          (                      n                      )                                                        +                                                            V                                              cs                        ⁢                                                                                                  ⁢                        0                                                              ⁡                                          (                      n                      )                                                                      )                            ×                                                                    T                    dem                                    ⁡                                      (                    n                    )                                                                                        T                    s                                    ⁡                                      (                    n                    )                                                                                                          (                  Equation          ⁢                                          ⁢          7                )            where Vcs0 represents the current-sensing voltage signal 172 when an on-time period starts during a switching cycle, Vcs1 represents the current-sensing voltage signal 172 when the on-time period ends during the switching cycle, and Rs represents the resistance of the resistor 130. In addition, n corresponds to the nth switching cycle, Vcs0(n) represents a magnitude of the current-sensing voltage signal 172 when an on-time period Ton starts in the nth switching cycle, Vcs1(n) represents a magnitude of the current-sensing voltage signal 172 when the on-time period ends in the nth switching cycle, and K represents the number of switching cycles that are included in the calculation. For example, K can be infinite; that is, the calculation of Equation 7 can include as many switching cycles as needed.
The output current 152 may thus be regulated based on information associated with the current-sensing voltage signal 172, the demagnetization process, and/or the switching period. However, the conventional current control schemes often suffer from low measurement accuracy.
Hence it is highly desirable to improve the techniques for current control of power conversion systems.